Automation of picking and placing objects by using industrial robots is well known. In a multi robot picking system, items and empty places are often moved by conveyors passing by the robots. Items can be picked and empty places can be filled while they are inside a robot's working range. Each robot receives information about new pick and place locations continuously during the picking process, e.g. retrieved by a camera.
The robots are normally operating independently. For every pick and place to be performed by a robot, a decision is taken in runtime which item that shall be picked or which empty place location that shall be filled. The decision is made among available items and empty places currently within reach by the robot. To make the robot work efficiently, it is important that matching items and empty places are available within reach at the same time.
The work load of an incoming flow of items or empty places can be shared between the robots. The incoming flow may be fluctuating or predefined, and the robots may share the work load. A fixed percentage of the items or empty places may be assigned to each robot. Alternatively, all items and empty places are assigned to all robots. If an item is picked, or an empty place is filled, by one robot, it becomes cancelled by the downstream robots.
A common system setup is to have two parallel conveyors passing multiple robots, wherein one of the conveyors is an item conveyor where items to be picked are transported, and one conveyor is an empty place conveyor where empty places are transported. The conveyors can move in the same direction i.e. concurrent flows, or they can move in opposite directions, so called counter flows. Typically, the velocity of the item conveyor cannot be affected by the system as it has a velocity that is predetermined and adapted to the amount of items it is transporting. However, the velocity of the empty place conveyor can often be affected or directly controlled by the system.
It is often desirable that all place locations are filled before passing by the last robot. It is also desirable that all incoming items are picked before passing by the last robot. Counter flow makes these combined tasks easier to achieve, due to the fact that the most upstream robot in respect of the picking flow has an overflow of incoming items that can be used to fill the remaining empty places. And the most upstream robot in respect of the empty place flow has an overflow of empty places that can be used to place the remaining items. In a parallel flow where both conveyors move in the same direction it is harder to achieve both tasks. The task of filling all place locations can be achieved e.g. by having a start/stop control of the empty place conveyor at the most downstream robot. The most downstream robot will stop the empty place conveyor if there is a risk that any empty place cannot be filled in time. However, it is hard to guarantee that also all items are picked unless the inflow of empty places and items are actively balanced and the velocities of the conveyors are similar, see for example U.S. Pat. No. 6,122,895 A.
When the conveyor velocities of the two conveyors need to be different e.g. due to a different density of items and empty places, there is a high probability that the most downstream robot now and then will have items available to pick but no empty places to fill, causing an overflow on the item conveyor. This problem can be solved e.g. by having a buffer of stationary empty places next to the robots. The buffer will be filled when no empty places are within reach on the empty places conveyor, and picked from when no items are within reach on the item conveyor. However, using a buffer will reduce the efficiency of the system since extra time is spent on moving items to and from the buffer.
US2014/0123606A1 describes a method and device for inserting individual products into containers in an automated line. An empty container is guided by the container supply as soon as a number of individual products corresponding to the number of products needed to fill a container are transported on the product belt.